Bench Terracing Concepts

Study Notes

Bench terraces are used in areas where climatic conditions favor the growth of certain cash crops.
They are also used for converting sloping lands to irrigated fields or for orchard plantations.
Types of bench terraces include:
- Hill-type bench terraces used in hilly areas with a slope reversely towards the hill.
- Irrigated bench terraces, featuring level benches for irrigation.
- Orchard bench terraces, narrower terraces for individual trees.
Bench terrace design factors:
- Average rainfall.
- Soil type.
- Soil depth.
- Average slope of the area.
- Purpose of construction.
Types of bench terraces based on slope:
- Sloping Outward: Used in low rainfall areas with permeable soils, with a shoulder bund for stability.
- Sloping Inward: Used in heavy rainfall areas, with a drain at the inward end to manage runoff.
- Level Top: Used in areas of medium rainfall, with deep and permeable soils, where most of the rainfall is absorbed by the soil.

Terrace spacing is determined by the vertical interval between two terraces.
Vertical interval (D) is twice the depth of the cut, aiming to balance cut and fill.
Depth of cut depends on soil depth and slope, avoiding exposure of bedrock and instability in embankments.
Terrace width (W) should match the intended use of the terrace.
Formulas are used to calculate the depth of cut based on the terrace slope:
- Vertical Cut: D/2 = W * S/100
- Batter Slope 1:1: D/2 = W * S/200
- Batter Slope ½: 1: D/2 = W * S/300
Terrace cross-section design considerations:
- Batter slope: For stability of fill or embankment.
- Shoulder bund dimensions: Larger for flat-topped terraces sloping outwards.
- Inward slope: Depending on soil conditions, with a drainage channel for runoff.
- Outward slope: Essential for terraces sloping outward to conserve rainfall.

Terrace alignment starts from the ridge and progresses towards the valley.
Contour lines are marked with leveling instruments, with the terrace width marked on the ground.
Local conditions like depressions, sharp turns, and field boundaries are considered during alignment.
Construction can start from the highest or lowest terrace based on soil conditions.
In areas where the topsoil is unsuitable, it is essential to keep the topsoil apart and spread it on the terrace.
Outlets are provided to safely dispose of runoff.
Waterways may be formed as outlets where natural options are unavailable.

The area lost for cultivation due to terracing is calculated based on the vertical interval (D), slope (S), and batter slope.
The formula considers the width available for cultivation after terracing and the width not available, representing the area lost.
The percentage area lost increases with steeper slopes and wider terrace spacing.

Focuses on protecting risers, outlets, and ensuring proper drainage.
Risers should be shaped and planted with grasses, particularly rhizome-type grasses.
Stones or hydro-seeding can be used for additional protection.
Outlets should be protected with sods of earth and blocks to prevent erosion.
Shoulder bunds should be planted with permanent vegetation.
Batter slopes should be stabilized with deep-rooted grasses.

Toe drains should be kept open and graded to ensure proper water flow.
Obstacles hindering water flow must be removed.
Grasses and weeds should be removed from the terrace surface.
Terraces should be reshaped after harvesting.
Ploughing should be done cautiously to avoid damaging toe drains and grade.
Risers should be maintained with healthy grass growth, free of weeds and vines.
Breaks in risers must be repaired immediately.
Cattle should be prevented from trampling on risers or grazing the grasses.
Outlets should be regularly inspected for protection and repairs.
Soil productivity is maintained through deep ploughing and ripping, green manuring, compost, and crop rotation.

Example calculates the length of terrace per hectare, earthwork required, and area loss for a 20% hill slope with a 2m vertical interval.
Calculations are performed for both vertical cut and batter slope of 1:1.
Formulas are used to estimate the width (W) of the bench terrace for different slope conditions.
The example illustrates how to determine the length of terrace, earthwork required, and area loss for specific design parameters.